This invention is concerned with a plunger suitable for use in a glassware forming machine, for example a glassware forming machine of the individual section type, in forming parisons of molten glass for subsequent formation into an article of glassware. Although the invention is applicable to other types of glassware forming machines, it is described herein in relation to a glassware forming machine of the individual section type.
Glassware forming machines of the individual section type are well-known. These machines comprise a number of individual forming units, called "sections", which all receive gobs of molten glass from a common source and feed their output to a common conveyor. Each section comprises at least one parison-forming mould in which gobs of molten glass are formed into parisons and at least one blow mould in which the parisons are blown to the required shape.
Parisons are formed in a mould cavity of a parison-forming mould of a machine of the individual section type either by a pressing operation or by a blowing operation. In either case, a plunger mechanism is used.
A conventional plunger mechanism comprises a cylinder disposed below the parison mould, and a piston movable in the cylinder towards or away from the mould cavity upon the introduction of fluid under pressure into the cylinder. A piston rod projects from the piston towards the mould cavity and is arranged to carry a plunger so that movement of the piston causes movement of the plunger towards or away from the mould cavity. In a press-and-blow process in which the parison is formed by a pressing operation and the parison is subsequently blown, the piston is moved by air pressure to move the plunger into the mould cavity so that the plunger presses the glass to the shape of the cavity. Air under pressure is then used to move the plunger fully out of the mould cavity to allow the parison to be removed. Finally, the plunger is moved to an intermediate position, either by a spring compressed as the plunger was withdrawn from the cavity or by an auxiliary piston and cylinder assembly, and the next gob of molten glass is introduced into the mould cavity on top of the plunger. In a blow-and-blow process in which the parison is formed by a blowing operation and the parison is subsequently blown again, the plunger is directly mounted on the piston rod and is surrounded by a thimble which is spring urged towards the mould cavity. A further spring acts between the thimble and the piston rod. In operation, air pressure urges the plunger into the mould cavity into a load position in which a gob of molten glass is dropped on top of the plunger. The air pressure is then removed and the further spring aforementioned moves the plunger away from the mould cavity to a counter-blow position in which air (called counter blow air) can be blown past the plunger into the mould cavity to cause a parison to be formed therein. Air pressure is then used to pull the plunger and the thimble away from the mould cavity, compressing both springs. This brings the plunger to a position in which it is clear of the mould cavity so that the parison can be removed.
Since the plunger of a plunger mechanism contacts hot glass which is a very abrasive material, it is subject to wear and requires frequent replacement. This is undesirable because the plunger is a complex and, therefore, expensive item to manufacture, being made of expensive material. The plunger must have a glass-engaging surface of complex shape, a fitment by which it is mounted on the piston rod, and must define air passages for cooling air or counter blow air and/or removal of air from the mould.
It is an object of the present invention to provide a plunger in which the portion most susceptible to wear can be easily replaced without replacing the remainder of the plunger.